With the Control Toolbox toolbar active, click on the Design Mode icon (triangle & pencil) so it is depressed, then click on your combo box so the sizing handles appear, then click on the Properties icon (finger pointing on sheet) so the properties window opens.
in the properties window, locate the line labled "ListFillRange" and enter the range for the options you want to appear in the combo box (something like A1:A7). Then on the "LinkedCell" line enter the cell in which you want the selection from the combo box to appear (something like C1).
then click the Design Mode button in the Control Toolbox so it is no longer depressed, and test your combo box.
HTH
Mark